Abstract

To our knowledge, there are no studies about the structure and ecological function of extrafloral nectaries (EFNs) in Opuntia robusta. This is the first description of EFNs in O. robusta, where young spines have an interesting structure and a secreting function, which are different from EFNs described in other Cactaceae species. We used light, scanning-electron, and transmission-electron microscopy to examine morphology, anatomy, and ultrastructure of the secretory spines in areoles in female and hermaphrodite individuals of O. robusta. Young cladodes develop areoles with modified and secretory spines as EFNs only active during the early growth phase. EFNs are non-vascularized structures, with no stomata, that consist of a basal meristematic tissue, a middle elongation region, and an apical secretory cone formed by large globular epidermal cells, containing nectar and medullar elongated cells. We observed the presence of Golgi apparatus, vesicles and plastids in the medullar and sup-epidermal cells of the spine. We propose that the nectar is stored in the globular cells at the apex of the spine and secreted by breaking through the globular cells or by pores. We recorded a more frequent presence of ants on younger cladode sprouts producing young secreting spines: this result is parallel with the predictions of Optimal Defense Hypothesis, which states that younger plant organs should be better defended than older ones because their loss produces a higher fitness impairment. Although Diaz-Castelazo’s hypothesis states that a more complex structure of EFNs correlates with their lower among-organs dispersion, comparing to less complex EFNs, non-vascularized structure of EFNs in O. robusta is not associated with their higher among-organs dispersion likened to O. stricta, which produces vascularized EFNs. We provide evidence that this characteristic is not a good taxonomic feature of Opuntia genus. Moreover, the comparison of EFNs of O. robusta and O. stricta suggests that the hypothesis of Diaz-Castelazo should be revised: it is rather a rule but not a law.

Highlights

  • Many plant species produce extrafloral nectaries (EFNs) which are secretory structures present in different organs such as leaves, petioles, stipules, and young stems [1]

  • The main goal of this study was to investigate the presence of extrafloral nectaries in young cladodes of O. robusta, and to explore their morphology and ultrastructure using light and electron microscopy in order to propose a mechanism of nectar secretion

  • We obtained the statement from the Ministry of Environment and Natural Resources of the United States of Mexico (Secretarıa de Medio Ambiente y Recursos Naturales) which stated that no permission is necessary for plants from Opuntia genus

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Summary

Introduction

Many plant species produce extrafloral nectaries (EFNs) which are secretory structures present in different organs such as leaves, petioles, stipules, and young stems [1]. Different types of EFNs may not be homologous: they share their glandular character [9] they can differ in their structures; i.e. they can have a form of single-cell secretory hairs, complex cups, shallow bowl-like depressions or formless glandular tissue. They can be highly vascularized or altogether lack vascularization [1]

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